How can I find holes in a mesh?
Use Select > Select All by Trait > Non-Manifold option in 3dView header (or Ctrl+Alt+Shift+M) and in options of operator located in Last Operator panel (they appear there after executing it) deselect everything but Boundaries.







Select all and choose Select > Select Boundary Loop. 



This will select boundary loops around selected edges.





Use shipped with Blender MeshLint addon (by default not activated). It allows choosing which categories of wrong topology to select, including non-manifold (however I think it will select wire, multiple faces etc. as well but still for completeness).